Stepper motor with integrated assembly

ABSTRACT

A stepper motor of the permanent-magnet sheet-metal type is constructed of a number of modules, simplifying assembly. The modular parts include a stator having integrally-molded winding bobbins and pole pieces. A rotor mounted in the stator includes a barrier member which separates a plastic, threaded hub molded interiorly from a permanent magnet molded exteriorly. A shaft having a first threaded end rotatably engaged with the interior of the rotor hub has a rectangular central portion extending through a rectangular opening of the stator to convert rotary motion of the rotor to linear motion of the shaft. An opposite end of the shaft includes a molded head and a tubular shield extending from the head. This shield and a tubular portion of the stator molding define a labyrinthal passage which inhibits the passage of contaminants into the rotor. A housing of the motor and a rotatably attached flange include engagable portions to lock the flange at a desired angular position.

This is a division of application Ser. No. 07/817,034, filed on Dec. 30,1991.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to stepper motors, and in particular topermanent-magnet sheet-metal type stepper motors.

2. Description of Related Art

Stepper motors include a stator for producing a rotating multipolemagnetic field and a multipole rotor for rotating about a central axisof the stator in synchronism with the rotating field. Typically therotating magnetic field is effected by applying electrical pulses ofcontinually changing phase to respective windings of the stator. Furtherdetails of typical stepper motor structure, operation and drivecircuitry can be obtained from a publication entitled AIRPAX StepperMotor Handbook published by Airpax Company, 604 West Johnson Avenue,P.O. Box 590, Cheshire, CT, U.S.A.

Because such a motor is capable of converting electrical pulses intodiscrete mechanical rotational movements, it is particularly useful forcontrolling the rotary or linear position of an object coupled to itsrotor. An example of one linear application which takes advantage ofthis capability is illustrated in FIG. 1. This Figure shows a steppermotor 10 which has been made by Airpax Company (part number A95228) forcontrolling the linear position of a shaped head 12 to control airflow14 in a throttle body 16 of an internal combustion engine (not shown).The external surface of the head 12 is precisely shaped to conform to aneck surface 18 of the throttle body so that the linear position of thehead regulates the magnitude of a clearance between the two surfaces.

Although it is not shown in FIG. 1, the head 12 is attached to a shaftwhich has a threaded end engaging an internal thread of the rotor in thestator contained in the motor housing 20. The shaft is prevented fromrotating with the rotor by means of a separate part which has fingersprojecting into respective longitudinal grooves in the shaft. Theseparate part is attached to an end of the stator, which itself includesa number of parts held together in two outer shell members which arefastened together This arrangement functions well, but is complicated toassemble Such complicated assembly of numbers of individual partstypifies the motor.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a stepper motor of simplemodular construction which may be easily and reliably assembled.

In accordance with a first feature of the invention the stator has acentral axis and comprises a monolithically-molded assembly including aplurality of multi-pole pieces embedded in a molding of insulatingmaterial. The molding defines at least one circumferentialwinding-holding channel disposed about the axis; an inner,axially-extending opening for receiving at least a portion of the rotor;a first axial end having bearing-holding means; and a second axial endcomprising a bearing portion having a bearing surface with a circularcross section and an adjacent shaft anti-rotation portion having aninner surface with a non-circular cross section of predefined shape. Therotor comprises a body including a first end supporting a bearingsecured by the bearing-holding means of the stator molding; a second endhaving a bearing surface disposed for rotation against the bearingportion of the stator; a permanent magnet portion disposed for rotationwithin the stator; and a threaded surface. The shaft comprises anaxially displaceable end extending from the stator and means forcooperating with the rotor and the stator to convert rotational movementof the rotor to axial displacement of the shaft. Such means include afirst portion of the shaft having a threaded surface disposed in therotor body in rotatable engagement with the threaded surface of therotor body and a second portion of the shaft including an outer surfacewith a cross section having the predefined shape and being slidablydisposed in the shaft anti-rotation portion of the stator molding.

In accordance with a second feature of the invention the rotor comprisesa generally cylindrical metallic barrier; a hub formed of a firstplastic material molded to an inner surface of the barrier; and a magnetformed of a second plastic material molded to an outer surface of thebarrier.

In accordance with a third feature of the invention the motor includes ahousing disposed around the stator and a connector means havingexternally-accessible terminals which are electrically connected to atleast one winding of the stator. The stator includes at least one bobbinincluding first and second sidewalls between which the at least onewinding is disposed. The first sidewall is located toward a first axialend of the stator, has a peripheral portion with at least one notch ofpredetermined depth, and the housing extends over the peripheral portionof the first sidewall. The connector means comprises an insulating bodymolded around the terminals. Each of the terminals has a first partwhich is accessible externally of the stepper motor and a second partextending through the at least one notch transversely of the firstsidewall. Each terminal is electrically connected to a wire forming theat least one winding. The predetermined depth of the notch issufficiently large to ensure that a clearance exists between the secondpart of the terminal and the housing.

In accordance with a fourth feature of the invention the motor alsoincludes a housing disposed around the stator and a connector meanshaving externally-accessible terminals which are electrically connectedto at least one winding of the stator. The stator comprises at least onebobbin having first and second sidewalls including respective plates ofmagnetically-permeable material between which the at least one windingis disposed. The housing comprises magnetically-permeable material andincludes a plurality of portions which are deformed into respectiveregions between the first and second sidewalls of the at least onebobbin to force the housing into contact with the plates and provide adecreased-reluctance magnetic return path for a magnetic field producedby said at least one winding.

In accordance with a fifth feature of the invention the motor includes ahousing disposed around the stator and a shaft coupled to the rotor suchthat rotation of the rotor effects linear motion of the shaft. The shaftextends axially out of an opening in a tubular end of the stator andincludes first and second ends. The first end is coupled to the rotorand the second end is attached to an operative member. The tubular endof the stator has an outer surface of predefined cross-sectional shape.A tubular member having the same cross-sectional shape extends axiallyfrom the operative member and around the tubular portion of the stator.The tubular member, the tubular end of the stator and the opening in thetubular end define a labyrinthal path that inhibits the passage ofcontaminants from outside of the motor into the rotor.

In accordance with a sixth feature of the invention the motor alsoincludes a housing disposed around the stator and includes means formounting the motor. The mounting means comprise a cylindrical endportion of the housing having a central axis and a mounting flangerotatably attached to the cylindrical end portion. The end portionincludes, in sequence, a first circumferential region including firstmeans for pressing against a rigid surface of an object to which themotor is to be mounted; a second circumferential region including secondmeans projecting from an outer surface of the cylindrical end portion toa predetermined radial distance from the central axis; and a thirdcircumferential region having a predetermined outer diameter. Themounting flange has a circular opening defined by a bevelled innersurface which increases in diameter from the third region toward thesecond region. The bevelled inner surface engages the second means andlocks the flange against rotation when the flange is axially forcedtoward the first region.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a typical application of a stepper motor having alinearly-movable shaft.

FIG. 2 is an exploded view, partly in cross section of a stepper motorconstructed in accordance with the invention.

FIG. 3 is a side view, largely in cross section, of the assembledstepper motor of FIG. 2.

FIG. 4 is a side view, in cross section, of a stator incorporated in thestepper motor of FIG. 2.

FIGS. 5a and 5b are end and side views, respectively, of a stator polepiece incorporated in the stator of FIG. 4.

FIGS. 6a through 6d are side views, in cross section, showing successivesteps in the manufacture of a rotor and bearing assembly incorporated inthe stepper motor of FIG. 2.

FIG. 7 is a side view of a shaft incorporated in the stepper motor ofFIG. 2.

FIG. 8 is a top view of a portion of the stepper motor of FIG. 3.

FIGS. 9a, 9b, 9c, and 9d are side views of end portions of electricalterminals shown in FIG. 8.

FIG. 10 is a cross-sectional view of the stepper motor of FIG. 3, takenalong the section 10--10.

FIG. 11 is a bottom, cross-sectional view of a housing and flangeportion of the motor of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 2 illustrates the component parts of a stepper motor constructed inaccordance with the invention. The parts, which are assembled along acentral axis x--x of the motor, include a shaft 22, a flange 24, ahousing 26, a stator 28, a rotor 30, a ball bearing 32, a elastomerO-ring seal 34, and an electrical connector 36. The assembled steppermotor is illustrated in FIG. 3.

The stator 28, which serves as a central building block of the steppermotor, is separately illustrated in FIG. 4. One of four identical poleplates incorporated in the stator is shown in end view in FIG. 5a and infront view in FIG. 5b. Each pole plate includes a number of integralpoles (six in this exemplary motor). The stator is amonolithically-molded assembly which is formed by injection molding aplastic material such as polybutylene terephthalate around the four poleplates 38a, 38b, 38c, 38d. The pole plates are arranged in two pairs38a, 38b and 38c, 38d, which are overmolded to form two respectivemolded bobbins 40 and 42. The two pole pieces are arranged with respectto each other such that their poles intermesh, but do not touch, as iswell known in the art.

The stator molding is hollow and includes respective portions definingan opening 44 at a first end for closely holding the bearing 32, acentral opening 46 within the pole pieces for receiving a central,magnetic portion of the rotor 30, and first and second functionalopenings 48 and 50 at a second end. First opening 48 receives acylindrical end 52 of the rotor 30 and has an inner surface whichcooperates with an outer surface of the end 52 to form a journalbearing. Second opening 50 slidably receives a portion 54 of the shaft22 and has a non-circular shape (when viewed along the axis x--x) whichcorresponds with that of the shaft portion 54. This shape must benon-circular to prevent rotation of the shaft with the rotor, and in thepreferred embodiment opening 50 and shaft portion 54 are substantiallysquare.

The rotor is illustrated in detail in FIGS. 6a through 6d, which showsuccessive steps in its manufacture. These steps are described in thefollowing, correspondingly-lettered paragraphs:

a. A generally-cylindrical barrier member 56 is cold formed from a metalsuch as aluminum. The use of the barrier member strengthens the rotorand enables different materials to be either simultaneously orsequentially molded to the inner and outer surfaces of the member. Thebarrier member includes an indentation 58 for preventing rotary andaxial movement of moldings formed on its inner and outer surfaces.Alternatively, a perforation may be forced through the barrier member.It also includes a reduced-diameter end portion 60 for receiving thebearing 32.

b. A hub 62 is injection molded within the barrier 56 from a plasticsuch as polybutylene terephthalate, with a protruding cylindrical endportion 52 (which is received in the stator opening 48 to form thejournal bearing). A thread 64 is molded into a central portion of thehub to rotatably engage a corresponding thread 66 on an end portion ofthe shaft 22. These engaging threads effect a linear displacement of theshaft along axis x--x when the rotor is rotated.

c. A highly-permanent-magnet material, such as a mixture of bariumferrite and a thermoplastic (e.g. nylon) is injection molded around theouter surface of the barrier member 56 to form a cylindrical permanentmagnet 68. The magnet is magnetized either during or after the moldingprocess in such a way as to permanently impress a number of polesdisposed radially along the magnet's periphery to form pole pairs. Inthis exemplary embodiment there are six pole pairs.

d. The ball bearing 32 is attached to the reduced diameter end 60 of thebarrier member. It is secured to the barrier member by bending the edgeof end 60 away from the axis x--x.

An embodiment of the shaft 22 is illustrated in detail in FIG. 7. Thisembodiment is substantially identical to that shown in FIGS. 2 and 3,except that a head 70a disposed at one end of the shaft has a slightlydifferent outer contour than a head 70b shown in those figures. The headis used in throttle body applications and the actual contour useddepends on the design of the specific throttle body in which it is used.

The shaft includes a central, threaded metal rod onto which the portion54 and the head 70(a,b) are injection molded from a plastic materialsuch as polyphenylene sulfide. Preferably portion 54 and head 70(a,b)are molded as a single unit, as is best shown in FIG. 2. The shaft alsoincludes a tubular metal shield 72 which has a funnel-shaped end portion74 embedded in the head 70(a,b). Preferably the end portion 74substantially conforms to the shape of the head, to prevent deformationof the outer surface of the head resulting from shrinkage during coolingof the molding. End portion 74 also includes a number (e.g., three) oftabs 76 which are punched inwardly from the metal shield, leavingperforations through which the plastic material forming the head flowsduring molding. The tabs strengthen the rigid connection between thehead and the shield, and the perforations ensure continuity of themolding material on opposite sides of the funnel-shaped portion 74.

The primary function of the shield 72 is to cooperate with the stator 28in preventing contaminants such as dirt and grime from entering themotor. As can best be seen in FIG. 3, the stator molding has at one endan outwardly extending portion 78 with the same shape as the shield,which is circular in the preferred embodiment. The portion 78 has anoutside diameter which substantially matches the inside diameter of theshield. The shield, in cooperation with the extending portion 78,presents a long labyrinthal path that inhibits the entrance ofcontaminants into the rotor. The end of the shield disposed in thehousing is flared outwardly to increase the length of the labyrinthalpath and to increase the strength of the shield.

Referring to FIGS. 3, 6d and 7, the shaft also includes a circularplastic portion 80 which is integrally molded onto the threaded rod withthe portion 54. Portion 80 includes an outwardly-projecting integralstop 80a which cooperates with a corresponding stop 82 molded onto aninner surface of the rotor hub 62. The two stops have axially-extendingfaces which are positioned such that they meet and stop rotor rotationif the shaft is drawn into the motor to a position where the flared endof the shield would otherwise be pulled against the proximal statormolding surface. This avoids excessive loading of the motor which couldbe required to drive the shaft out of the stator if the shield bindsagainst the stator molding.

FIGS. 3, 4 and 8 illustrate attachment and electrical connection of theconnector 36 to the stator 28. The connector is a molding of electricalinsulating material, such as polyethylene terephthalate in which fourelectrical terminals 84a, 84b, 84c, and 84d are embedded. The terminalspass through the molding from an access port 86, for receiving a matingelectrical connector (not shown), to a face which lies adjacent asidewall of stator bobbin 42. This sidewall, in which the pole plate 38dis embedded, is notched in a region 88 to provide a receiving space forends of the terminals. The integrally-molded sidewalls of the twobobbins, in which the pole plates 38b and 38c are embedded, are alsonotched in a region 90 to provide a passage to the terminal ends for theends of a first winding 92 of insulated magnet wire wound on the bobbin40. Similarly, a second winding 94 is provided on bobbin 42.

The method of attaching the connector and making the electricalconnections will be made clear by referring to FIGS. 2, 8 and 9. Thefacing surfaces of the connector 36 and the stator 28 are affixed toeach other, preferably by ultrasonic welding. Initially, the ends of theterminals are oriented substantially parallel to the facing surfaces,extending away from the axis x--x, to facilitate electrical connectionof the windings. As is illustrated in FIG. 9, the terminal ends haverespective thickened portions 96a, 96b, 96c, and 96d for preventing thewire ends of the windings from slipping off of the terminals.

Each of the wire ends is first wrapped around a portion of therespective terminal disposed between the connector body and thethickened portion, and is then wrapped around the distal end of theterminal. The terminal ends and the attached wires are then dip solderedor welded and bent down into the notched region 88 far enough to avoidcontact with the winding and to provide an electrically-insulatingclearance between the bent terminal ends and the housing, when it isattached to the stator. There is no possibility of the terminal ends orthe end wires of winding 92 contacting pole plates 38b, 38c or 38d,respectively, because each of these pole plates is oriented in thestator molding with a respective flattened portion 98 located under theregions where the notched areas are formed. (See FIG. 5a)

As is illustrated in FIG. 3, the housing 26 is assembled over the stator28 and over one end of the electrical connector 36 which includes aperipheral portion defining a channel in which the elastomer 0-ring seal34 is disposed. The housing is secured by bending an end portion 100over the peripheral portion. It has been found that the operatingefficiency of the motor is significantly improved by deforming smallportions of the housing, made of a metal of good magnetic permeabilitysuch as low-carbon steel, into spaces above the windings to ensure thatthe housing contacts each of the pole plates. Only two of theseindentations 102, 104 are visible in FIG. 3, but in the preferredembodiment several (e.g. three) such indentations 104 (see FIG. 10) areformed in the housing around each winding. This arrangement provides lowreluctance return paths for the magnetic fields produced by the windingswhen they are energized.

As is best seen in FIGS. 10 and 5a, the housing further includes atleast one indentation 106 which is located to engage aligned,corresponding indentations 108 in the embedded pole plates 38b and 38cto secure the housing against rotation with respect to the stator. It isnot necessary that all of the indentations 108 in the pole plates beutilized.

FIG. 11 illustrates mounting means by which the stepper motor isattached to a body, such as the throttle body of FIG. 1, at any desiredrotational position around the axis x--x. The mounting means includesthe flange 24 and a cylindrical end portion of the housing 26, which areboth shaped to secure the motor to the body at the desired rotationalposition.

The cylindrical end portion includes a first circumferential regionhaving a number of projecting indentations 110, a second circumferentialregion having a number of projecting indentations 112, and a thirdcircumferential region 114 having a smooth outer surface of radius R.The indentations 110 and 112 are evenly spaced around the cylindricalend portion, e.g. at intervals of sixty degrees.

The flange 24 has a circular opening defined by a bevelled inner surface116 which increases in radius from a magnitude slightly greater than R,where it contacts the third circumferential region 114, to a radiusslightly greater than the distance by which the indentations 112protrude from the axis x--x. The indentations 110 are dimensioned andpositioned to press against a circumferential surface 118 in the bodywhen the flange 24 is pressed against an outer surface of the body andmounted by means such as bolts (not shown) passing through holes 120 and122. This action also forces the projecting indentations 110 intocircumferential surface 118 of the body and the projecting indentations112 to compress against the bevelled surface 116 of the flange.

Before the indentations 112 and 110 are forced into the respectivesurfaces 116 and 118, the flange is free to rotate about the housing.This enables the flange to be readily adjusted to any angular positionwhich will make the access port 86 of the connector 36 accessible to amating connector. After the indentations 112 and 110 are forced into therespective surfaces 116 and 118, nor further rotation is possible. Theindentations 112 and 110 projecting from the cylindrical end portion actas spring members when forced against the respective surfaces of theflange and the body, thus allowing repetitive mounting to the bodywithout losing the locking ability.

We claim:
 1. A stepper motor including a stator having a central axisand at least one winding disposed about the axis for producing aplurality of magnetic poles in the stator, a permanent-magnet rotorhaving a plurality of magnetic poles and being mounted for rotationabout the axis, a housing disposed around the stator, and connectormeans having externally-accessible terminals electrically connected tothe at least one winding, characterized in that:a. the stator comprisesat least one bobbin having first and second sidewalls includingrespective plates of magnetically-permeable material between which theat least one winding is disposed; and b. the housing comprisesmagnetically-permeable material and includes a plurality of portionswhich are deformed into respective regions between the first and secondsidewalls of the at least one bobbin to force the housing into contactwith the plates and provide a decreased-reluctance magnetic return pathfor a magnetic field produced by said at least one winding.
 2. A steppermotor as in claim 1 where at least one of said sidewalls includes acircular peripheral portion having a radially extending deformation,said housing is cylindrical and includes a mating deformation to preventrotation of the housing relative to the stator.
 3. A stepper motor as inclaim 1 or 2 where the connector means is attached to an axial end ofthe stator and where the housing has a first axial end portion deformedover a corresponding portion of said connector means to secure it withinthe housing.
 4. A stepper motor as in claim 3 where the housing has asecond axial end portion which is deformed over a corresponding portionof the stator to secure it within the housing.